Pulsed laser or LED diode drivers are used to generate pulses of current, typically into a series array of lasing diodes. The light output is used for various purposes, such as pumping lasers, or timed illumination. The driver design typically comprises a storage capacitor, the laser diode array, and a pulsed current source, connected in series. When the current source is turned on, energy is drawn from the capacitor through the diode array. The voltage on the capacitor falls, so the current source must have sufficient compliance to continue to operate as voltage falls. For good efficiency, a low voltage loss across the current source is desired, but this requires a large and bulky capacitor to minimize voltage sag.
It is desirable to efficiently pulse a laser diode or light-emitting diode for a number of applications. Using a linear current source is inefficient because voltage is dropped across the linear pass element, or shunt element as shown in commonly-owned U.S. Pat. No. 6,587,490. Turning a single phase switching power supply off and on from zero is problematical because the control loop bandwidth limits the potential switching speed. The control loop is slower than the switching frequency to work correctly. The proposed approach allows good pulsed current waveforms with greater than 90% efficiency possible.
Analog constant current sources or pulsed analog constant current sources using linear dissipative pass elements have been used as diode drivers to power light emitting diodes (LED's), often laser diodes. FIG. 1 illustrates an array of LEDs (or diode array) that are connected to a power source. A linear control (pass) element is disposed in the return path from the LED array to the power source. Current flowing through the LEDs flows through a current sense resistor which supplies a voltage indicative of current to an input of an error amplifier, the other input of which receives a reference demand voltage indicative of the desires current. The output of the amplifier controls the linear control element to maintain a constant current through the LEDs. This is a simple, straightforward analog control loop. Such analog current sources are inefficient due to power (e.g., heat) dissipation in the linear pass element controlling the current.
Commonly-owned U.S. Pat. No. 6,697,402 shows a method to turn on a diode quickly by storing current in an inductor. Commonly-owned U.S. Pat. No. 6,587,490 shows a dissipative linear shunt bypass regulator. The applicant's (Analog Modules, Inc.) own Model 779A “High Power CW or Pulsed Laser Diode Driver” shows a typical linear driver (see website of Analog Modules, Inc. for Data_sht/779a.pdf). U.S. Pat. No. 5,287,372 (Ortiz) shows a quasi-resonant diode drive current source.